Blockpc

This patch addresses issue #81 by implementing blockpc based on Peter's
suggestion. blockpc has the same semantics as pc, except that pc only applies
when the given predecessor of the given block is chosen.

blockpc(s) are collected in a way similar to what we currently do for pc(s),
and are encoded with a similar technique for encoding UB constraints.

The parser is also updated to parse blockpc(s).

I am sorry the path is a bit big...

You should be able to combine `GetRelevantPCs` and `GetRelevantBPCs` into a single function that returns a tuple (PCs, BPCs). Either that or write them as functions that take a set of equivalence classes with `Cand` already added to it.